Fast optical receiver

ABSTRACT

A method for optical transmission of high speed data and an optical receiver for receiving such high speed data is provided. The method includes transmitting an optical signal having a first logic-high and first logic-low defining a first modulation amplitude that is sub-band modulated with a toggling signal having a first toggling amplitude with a first modulation index, receiving the optical signal with an optical receiver circuit and converting the optical signal to an intermediate electrical signal, IES, having: a second logic-high and a second logic-low defining a second modulation amplitude, and a second toggling amplitude having a second modulation index, providing a decision threshold relative to the IES as a function of the second modulation amplitude, and adjusting the threshold by determining the second toggling amplitude and adjusting the threshold relative to the IES based on proportionality between the second toggling amplitude and the second modulation amplitude.z.

TECHNICAL FIELD

The invention relates to a method for optical transmission of high speeddata and an optical receiver suitable for receiving such high speed datafor example in an optical interconnect.

BACKGROUND ART

A requirement for conventional binary data transmission is theestablishment of a well-defined logic threshold. Metallic wired systemsuse pre-defined dc logic levels for this purpose. This is unsatisfactoryin an optical system where absolute signal levels are commonly not knowna priori. The conventional solution is ac-coupling between the receiverand the logic quantizer. With this approach, dc logic threshold levelsare established by forming a “signal average” of the second data pulse.Signals above the average are considered as logic ONEs, while signalsbelow the average are logic ZEROs. While ac-coupled receivers may workwell for encoded continuous data transmission they do not work well forburst mode data transmissions and signals where the time average of thesignal may vary.

A well known problem in the art of digital optical communication is thedifficulty in using unencoded data, where the data to be transmitted isallowed to have long strings of only ONEs or only ZEROs also referred toas long CIDs (Consecutive Identical Digits) and/or the data is notnecessarily DC balanced (i.e. having on average substantially equalamounts of the high logic and low logic values). This is because, forproper operation of an AC coupled receiver, the optimal threshold isusually substantially midway between the logic high and logic low valueand should substantially correspond to the average (for brevity “logichigh” and “logic low” may be referred to as simply high and low values).Therefore non-transition periods, i.e. a period of long CIDs should besmall relative to the time constant determined by the capacitance forcoupling between a) a preamplifier of an electrically-converted versionof the optical signal and b) a comparator which is used to determine thelogic value, i.e. 1 or 0, of the incoming optical signal. In opticallinks this problem is commonly solved by using encoding of the signalprior to transmission and decoding after reception. The encoding ensuresthat transition occurs in the optical data signal even if the data is along string of CIDs. Furthermore, the encoding often ensures that theoptical signal is balanced. Disadvantageously, doing so mandates that adecoder be present in the receiving system to remove the formatting andreconstruct the original data. Also, data transmission efficiency iscommonly degraded because of the required extra bits for the encoding.

While AC coupling is not always needed in electrical communicationsystems, it was believed to be required in optical systems, becausethere is no common electrical connection between the transmitter and thereceiver. Nevertheless, eventually some of the above-noted disadvantageswere overcome by burst-mode digital optical receivers which use directcurrent (DC) coupling. Such a DC coupled burst-mode digital opticalreceiver is disclosed in U.S. Pat. No. 5,025,456 which was intended toadapt to the amplitude of the incoming burst data packet andautomatically adjust the logic threshold voltage to the dc center,ideally during the first bit of the input data burst. The inventorslater characterized the purpose of this invention as meeting thechallenge of having several transmitters on the same optical bus wherethe power levels may vary dramatically between transmitters, see U.S.Pat. No. 5,430,766. This later patent aimed at solving the problem ofhaving a relatively high amount of constant light from each transmittercombined reaching the receiver. The same inventors also patented a burstmode receiver in U.S. Pat. No. 5,371,763. The above cited patents areincorporated herein by reference.

However, one drawback of these prior arts burst-mode receivers is thatthey rely on the use of a preamble preceding the payload data in a databurst to set the logic threshold. Such a preamble will often comprise adata sequence aimed at initializing the receiver rather than carry data.Such receivers are therefore often not suitable for receiving unencodeddata because correct determination of the first digits after a long CIDmay not be ensured at least not with good pulse width distortion.

DISCLOSURE OF INVENTION

Today, optical interconnects or optical cables appear to be increasinglyimportant means for transportation of digital information. These linksare commonly expected to enable high transfer speed of e.g. 10 Gbit/s orhigher. To facilitate simple integration into the system it would oftenbe an advantage to enable the link to robustly carry a data stream whichdoes not require excessive coding which reduces transfer speed. Inparticular it is desired to enable transmission of a less encoded oreven an unencoded data stream; i.e. without requiring the use ofpreambles and/or encoding to ensure a balanced data stream that wouldotherwise allow the link to set a suitable threshold before any payloaddata is received. The link should advantageously be able to toleratelong durations of no data i.e. long series of consecutive identicaldigits (CIDs) as well as other deviations from a balanced signal. At thesame time the receiver should preferably be capable of correctlyinterpreting the first bit of a data stream received after a period suchas even a period of. several days of no data, optionally even withouttransmission of a preamble. During such a time drifting in thetransmitter caused by e.g. temperature and/or aging of the laser mayoccur or the optical loss from transmitter to receiver may change.Compared to many prior art systems the challenge of providing a suitablelogic threshold (here also referred to as just “threshold” for brevity)for the first bit has increased due to the increased transfer speed andhence reduced time allocated to adjust the threshold during this bit.There is therefore a need for an optical receiver which is capable ofrobustly receiving high speed data which is advantageously substantiallyunencoded.

In co-pending U.S. application Ser. No. 13/152,053, which is herebyincorporated by reference, this challenge was addressed by realizingthat while laser diodes, in particular VCSELs, tend to drift infunctionality due to temperature and/or aging it is possible via priorart methods to construct the driver to compensate for this to an extentso that at least most of the time the extinction ratio in a binarysignal is substantially constant or only varies slightly such as forexample 0.5 dB. In an embodiment of that invention the receiver directlyor implicitly uses a previously determined extinction ratio and the peakvalue that is received as the CID to set the threshold of the receiver.An object of the invention is to provide an alternative method suitablyfor optical transmission of high speed data and an optical receiversuitable for receiving such data with high accuracy (low loss ordistortion of data), even where the data comprises long periods of CID.where the required encoding is very low and advantageously the data issubstantially unencoded.

This and other objects have been solved by the invention as defined inthe claims and as described herein below.

It has been found that the invention and embodiments thereof have anumber of additional advantages which will be clear to the skilledperson from the following description.

In an embodiment, the invention in particular provides a novel method ofsetting the threshold of the invention even when the transmitted datacomprises long periods of CID.

In an embodiment the required encoding is very low and advantageouslythe data is substantially unencoded.

The term “substantially unencoded” is used to include slightly encodeddata which according to the invention advantageously could have beentransmitted and received without such slightly encoding by the method ofthe invention. Advantageously the data is fully unencoded.

According to the invention the method has been found to enable highspeed optical transmission of data with low loss or distortion of dataeven when subjected to DC drift. In an embodiment the transmitted datacomprises data which is not DC balanced.

In an embodiment the data is not coded to reduce or remove long CIDsduring optical transmission.

In an embodiment the present invention relates to a method for opticaltransmission of data in form of at least one optical signal comprising abinary content, the method comprising

-   -   transmitting the optical signal with the binary content        comprising a first logic-high and a first logic-low defining a        first modulation amplitude wherein the first modulation        amplitude is sub-band modulated with a toggling signal having a        first toggling amplitude with a first modulation index relative        to the first modulation amplitude,    -   receiving the optical signal with an optical receiver circuit        and converting the optical signal to an intermediate electrical        signal, IES, the IFS having the following parameters:        -   i. a second logic-high and a second logic-low defining a            second modulation amplitude and        -   ii. a second toggling amplitude having a second modulation            index relative to the second modulation amplitude,    -   providing a decision threshold relative to the IFS as a function        of the second modulation amplitude, and    -   adjusting the decision threshold by determining (e.g. by        measuring) the second toggling amplitude and adjusting the        decision threshold relative to the IES based on proportionality        between the second toggling amplitude and the second modulation        amplitude.

By the method of the invention it has been found that the receiver in asimple way can determine the threshold by determining theproportionality between the second toggling amplitude and the secondmodulation amplitude. Thereby the need in prior art technology forcoding the signal is highly reduced or even avoided.

The term “first” about a parameter is used to indicate that it is aparameter of the signal about to be transmitted, whereas the term“second” about a parameter is used to indicate that it is a parameter ofa signal that has been received and optionally converted to anelectrical signal e.g. IES.

The term a first logic high means a first logic high signal value of amodulation amplitude and likewise them on a first logic-low means afirst logic low signal value of a modulation amplitude.

The first modulation amplitude of the binary signal is the modulationamplitude of the signal to be transmitted and is the difference betweenthe first logic high and the first logic low values. This firstmodulation amplitude can be selected to have any value within theoperation range of the optical transmitter used.

The phrase “an optical signal comprising a binary content” is in thefollowing also referred to as “a binary optical signal”.

The phrase “the first modulation amplitude is sub-band modulated with atoggling signal having a first toggling amplitude with a firstmodulation index relative to the first modulation amplitude” means thatat least some of the binary signal is sub-band modulated to obtain thefirst toggling amplitude, wherein the toggling amplitude is differentfrom the first modulation amplitude.

The term “determine or determining” of a parameter means in anembodiment measuring the parameter e.g. measuring a value and/or anamplitude. In an embodiment the parameter in question is determinedusing one or more other measured parameters.

The first modulation index is advantageously obtained as the firstmodulation amplitude divided with the first toggling amplitude e.g.provided in percentage.

The decision threshold is also referred to as the logic threshold orsimply the threshold.

Optionally the decision threshold is determined as a function of thesecond modulation amplitude plus a bias.

In an embodiment the decision threshold is determined as a function of afraction of the second modulation amplitude plus a bias.

In an embodiment the method comprises at least sometimes adjusting thedecision threshold by: measuring the second toggling amplitude andadjusting the decision threshold relative to the IES based onproportionality between the second toggling amplitude and the secondmodulation amplitude.

In this way the receiver may use the magnitude of the toggling amplitudeto predict the modulation amplitude. In the event of a long series ofCIDs, i.e. the receiver receives a long series of e.g. logic-low values;the second logic-low and the second toggling ratio provide sufficientinformation to determine where the logic-high will be when the series ofCIDs ends. In an embodiment the receiver is therefore able to set thethreshold correctly during such series of CIDs and interpret the firstbit correctly after such a period even at high data rate with little orno pulse width distortion. I.e. the receiver recovers the binary signalcontent and converts the IES in the receiver circuit into a binaryelectrical signal output downstream of the input via the decisionthreshold. Hence, the invention will enable optical receivers forsubstantially or fully unencoded data at high data rates with reduced orno constraints on minimal bandwidth of the signal, i.e. even if thebinary content comprises long series of CIDs. In the event of a changein the loss in the transmission from transmitter to receiver, the secondmodulation index will likely be constant as both modulation and togglingamplitudes are often equally affected.

Advantageous the laser used for transmitting the signal is operating inits linear regime of output. Thereby the first modulation index can beheld substantially constant.

As explained above the invention can in an embodiment be used fortransmitting and receiving unencoded data. However, the invention canalso be applied for transmitting and receiving coded data. Even wherethe data is coded large advantages can be obtained by the inventionbecause the coded data need not be coded for ensuring low loss ordistortion of data, but can for example be coded for any other reasons.

In an embodiment the method of the invention is applied for transmittingcoded data, or partly coded data, i.e. where only some parts e.g. a bitlength of the data is coded. The coding is for example a scrambling ofdata or an encryption of data.

In an embodiment a VCSEL (vertical-cavity surface-emitting laser) isused for transmitting the signal and the VCSEL is modulated over asubstantially linear regime of output intensity to current so that evenif the slope of this curve changes (e.g. due to changes in temperatureor aging), the modulation index is constant. Note that in an embodimenta series of CIDs of one logic value does not exclude the presence of afew bits of the other logic value. This is discussed in more detailbelow.

In an embodiment modulation index is defined as the ratio of thetoggling signal relative to an average value of the modulation amplitudeover a period of the toggling signal. In an embodiment the togglingamplitude is distinguishable from signal content by having a smalleramplitude, so that the modulation index is less than 50%, but oftenpreferably less than 40%, such as less than 30%, such as less then 20%,such as less than 15%, such as less than 10%, such as less than 7%, suchas less than 5%, such as less than 2%, such as less than 1%, such asless than 0.5%. On the other hand the toggling signal should also bedistinguishable from the noise floor. This is in an embodiment easier ifthe toggling signal is substantially binary with a low center frequencyso that the same value is maintained for a relatively long time which inturn may allow averaging. On the other hand, it is expected that aminimum value of the modulation index is required such as more thanabout 0.5%, such as more than about 1%, such as more than about 2%, suchas more than about 5%.

Note that in an embodiment the first modulation index, i.e. the ratio ofmodulation amplitude to toggling amplitude, need not be strictlyconstant as long as it is predictable for the receiver and/or constantwithin a tolerable margin.

In an embodiment the present invention find application for links usingburst mode transmission where a payload of data is preceded by apreamble. The invention can for example provide the receiver with animproved initial value for the threshold improving an initialization ofthe receiver based on the preamble.

In an embodiment the invention relates to a receiver circuit arranged toimplement the method of the invention. In one such embodiment theinvention relates to an optical receiver circuit for receiving a binaryoptical signal and converting it to an intermediate electrical signal,IES, with a binary content comprising

-   -   a second logic-high and a second logic-low defining a second        modulation amplitude, and    -   a sub-band modulation of the second modulation amplitude having        a toggling amplitude,    -   the optical receiver comprising a circuit arranged to measure an        toggling amplitude of the second toggling signal, and a circuit        arranged to adjust the decision threshold based on the second        toggling amplitude.

The invention also relates to corresponding receivers, i.e. receivercircuits according to the invention connected to a photo detector, suchas a photo diode.

In an embodiment the invention relates to a transmitter and a drivercircuit suitable for driving a light source so that an optical binarysignal as described above is obtained.

In an embodiment the optical binary signal is obtained from anelectrical binary signal with the first modulating amplitude, which hasbeen subjected to a sub-band modulation with the toggling signalfollowed by converting the signal to an optical signal.

In an embodiment the optical binary signal is obtained from anelectrical binary signal with the first modulating amplitude which isconverted to an optical signal while it simultaneously or after theconversion has been subjected to a sub-band modulation with the togglingsignal.

In an embodiment the toggling signal has a first toggling amplitude andperforms a sub-band modulation of the first modulation amplitude suchthat the first modulation amplitude alternates between at least 2 valuesdetermined by the toggling amplitude. The first modulation index canhave any value positive value with the exception of 1.

In an embodiment the invention relates to an optical transmittercomprising a drive circuit comprising an output stage arranged to drivea light source with a waveform based on a binary input signal so thatthe light source (during use) transmits a binary optical signal having afirst modulation amplitude, a first logic-high and a first logic-low,the transmitter further comprising a toggling circuit arranged so thatthe modulation amplitude is modulated with a toggling signal having afirst toggling amplitude preferably having a substantially constantratio with the first modulation amplitude. In an embodiment such atransmitter is realized by applying a conventional chip for driving alight source, the chip has a programmable modulation and bias current.The toggling signal is then obtained by sequentially programming themodulation current to obtain the toggling signal e.g. using amicrocontroller. Depending on the design of the drive chip, programmingof a bias current may also be desired to obtain a modulation of bothlogic-high and logic-low.

In an embodiment of the optical transmitter drive circuit according tothe invention, the binary optical signal is obtained by coupling thelight source to a Si waveguide and modulating light from the lightsource by coupling the binary input signal to the waveguide andsimultaneously coupling the coupling the toggling circuit to thewaveguide, such that the binary input signal and the toggling signalprovided by the toggling circuit is driving the light source with awaveform such that the light transmits a binary optical signal having afirst modulation amplitude, a first logic-high and a first logic-low,and such that the modulation amplitude is sub-band modulated with thetoggling signal to have a first toggling amplitude.

Advantageously the light is separated into at least two branches in thewaveguide and the binary input signal as well as the signal from thetoggling circuit is coupled to the waveguide in form of modulatedvoltage to provide a modulated delay of some of the light e.g. onebranch of light. The branches of light are thereafter combined toprovide the modulated binary optical signal. Due to the delay of thelight in one or more branches, the delayed light will be out of phasewith light in one or more other branches where the light has not beendelayed or where it has bee delayed to a different degree. The delayedlight will therefore when merged with light in one or more otherbranches, therefore more or less extinguish the light, and thereby sincethe light is delayed in a modulated fashion, the resulting light will ina similar way be modulated with respect to light intensity. To avoidfully light extinguish, the amount of light subjected to the modulateddelay may advantageously be less than the amount of light which is notsubjected to the modulated delay.

In an embodiment the light is separated into at least two branches inthe waveguide and the binary input signal is coupled to the waveguide inform of modulated voltage to provide a modulated delay of some of thelight e.g. one branch of light and the signal from the toggling circuitis coupled to the waveguide in form of modulated voltage to provide amodulated delay of some of the light e.g. the other branch of light. Thebranches of light are thereafter combined to provide the modulatedbinary optical signal.

In an embodiment the waveguide is a wire waveguide e.g. as described byK. Yamada in “Silicon Photonics. Topics in Applied Physics 119, 1-29,Springer (2011).

In an embodiment the application may provide that series of long CIDsonly occur as long series of logic-low or highs, in which case it may besufficient to only modulate that logic value in the transmitter. In anembodiment the toggling signal is provided by a circuit integrated withthe drive circuit.

In an embodiment the modulation of the modulation amplitude issignificantly slower than the bit frequency of the signal which maysimplify efficient separation of the toggling signal from the signalcontent. Accordingly, in an embodiment the binary optical signal has abit frequency and the toggling signal has a center frequency of lessthan 25% of the bit frequency such as less than 10% of the bitfrequency, such as less than 5% of the bit frequency, such as less than2.5% of the of the bit frequency, such as less than 1% of the bitfrequency, such as less than 0.5% of the bit frequency, such as lessthan 0.1% of the bit frequency, such as less than 0.01% of the bitfrequency. However, it is in principle possible to use faster centerfrequency of the toggling signal.

As noted above, the present invention has shown to provide a significantadvantage particularly at high bit rates such that in an embodiment thebit frequency of the optical signal content is larger than 100 Mbit,such as larger than or equal to 1 Gbit, such as larger than or equal to10 Gbit, such as larger than or equal to 25 Gbit, such as larger than orequal to 50 Gbit, such as larger than or equal to 100 Gbit.

DETAILS OF THE INVENTION

The second toggling amplitude may be applied in a variety of ways todetermine a suitable decision threshold. In an embodiment the secondmodulation index is measured as a part of an initialization and/orduring periods where the signal content allows measurement of the secondmodulation amplitude (MA) and the second toggling amplitude so that theratio (i.e. the second modulation index) may be determined. After thesecond index has been determined, the threshold can be adjusted bymeasuring the toggling ratio which provides information of the secondmodulation amplitude at least when the second modulation index may beassumed constant. Updating the second modulation index during operationmay be advantageous if the second index is expected to change duringoperation. In an embodiment an activity detector (discussed below) isapplied to determine when the binary content of the IES (The IES binarycontent) is suitable for measuring the second modulation index. In anembodiment the threshold is substantially always updated based on thesecond toggling amplitude whereas in an embodiment another approach maybe applied when allowed by the IES binary content. For example, the IESbinary content may in periods be substantially balanced in which case anaverage may be applied during such periods. In the alternative or incombination with this example, the receiver adjusts the threshold usingpeak detection in periods when the IES binary content is so that theoutput from peak detectors may be trusted to reflect the true valuesufficiently.

In an embodiment the toggling signal may be used in the receiver. Theuse of the toggling signal in the receiver comprises advantageouslysetting the second modulation index as a predetermined value, i.e.assuming a modulation index at the receiver and adjusting the decisionthreshold based on determining the second toggling amplitude. Such anapproach may in an embodiment be preferred where the modulation index ofthe receiver is known and/or when the accuracy of the threshold is lesscritical.

In an embodiment the method comprises adjusting a gain in the receivercircuit, e.g. via a feedback loop, so that the second toggling amplitudeis substantially constant. This is exemplified in the attached examples.

In an embodiment the method further comprises amplifying the IES therebyadjusting the second modulation amplitude and second toggling amplitude.

In an embodiment setting of the threshold further comprises measuring atleast one of the following values

-   -   a. a P_(H) value related to the second logic-high,    -   b. a P_(L) value related to the second logic-low, and    -   c. a MA value related to the second modulation amplitude and        determining the validity of the measured values.

In an embodiment MA is formed by the difference of P_(H)−P_(L) butalternative ways to measure or determine the modulation amplitude may beapplied. In the context of this text the phrase a value “related to”another value or parameter infers a functional relationship between twoparameters. In an embodiment “relates to” indicates the possibility of ascaling and/or offset which may be suitable in one implementation of theinvention. For example, in an embodiment a value may be determined as avoltage whereas the other is a current or vice versa. For example, thevoltage obtained from the current through a resistor, where an offsetmay be caused by another current flowing through the resistor and/orcaused by that the voltage across the resistor refers to a potentialother than ground e.g. the positive supply. In one such embodiment themeasured values may differ by a proportionality factor. In an embodimentthe photo detector is not a linear device and the binary electricalsignal is therefore not proportional to the optical signal. In suchembodiment P_(H), P_(L) and/or MA are compensated for this non-linearitye.g. via a look-up table. In such embodiment the binary electricalsignal is understood in the embodiments described herein to be theelectrical signal that would have occurred had the photo detector beenlinear. In an embodiment “a value related to” a second value is taken tomean substantially equal to or in an embodiment equal to.

Depending on the construction of the circuits for measuring at least oneof P_(H), P_(L) and MA these measurements may deviate from the actualvalue. In an embodiment the measured values are pattern dependent, i.e.depend on the signal content (see e.g. discussion of peak detectorsbelow). Depending on the signal content and the design of the circuitssome signal content may therefore render the output from one or more ofP_(H), P_(L) and MA invalid. For example a long series of CIDs oflogic-high may render measurement of P_(L) and MA invalid in which caseembodiments of the present invention will instead depend on the validparameters (in this example P_(H)) and the toggling amplitude applied tothe logic high value. Different examples where measured values areinvalid and how to detect this are discussed further below.

The decision threshold (or simply the threshold) is a threshold fordeciding if a signal value is a logic-low (0) or a logic-high (1). Wherea signal value is above the decision threshold is determined to belogic-high (1) and where a signal value is below the decision thresholdis determined to be a logic-low (0).

Advantageously the decision threshold is provided as a function of thesecond toggling amplitude below the second logic high and above thesecond logic low. This function can e.g. be a multiple of below 1,advantageously between 0.3 and 0.7 of the second toggling amplitude.

In an embodiment the decision threshold is set or adjusted based on themeasured values when the measured values are determined to be valid. Inan embodiment the threshold is set as a function of a fraction of thesecond modulation amplitude optionally shifted by a bias, i.e. aconstant value. In one such embodiment the threshold is set to a valuebased in the present P_(H) and present P_(L) when both present P_(H) andpresent P_(L) are deemed valid. In an embodiment the threshold is set asa value related to the center value between P_(H) and P_(L). In anembodiment the transmitter may (as an example) exhibit nonlinearity sothat a DC balanced signal content does not produce a DC balanced opticalsignal. In one such embodiment or other embodiments it may be preferableto set the threshold to a value related to (P_(H)+P_(L))/x where x maytake values different from 2. It is also possible to set the thresholdbased on MA. For example, the threshold is in an embodiment set to afraction of MA optionally plus a bias, e.g. as MA/y plus P_(L) when atleast P_(L) is valid and as P_(H)−(1−1/y)·MA when only P_(H) is valid.In one such embodiment y is arranged so that the threshold, TH_(P), is<TH_(P)<P_(H). As noted above, MA may be found based on theproportionality with the second toggling amplitude i.e. as function ofthe second modulation index divided by the second toggling amplitude.

Whether to use P_(L)+MA/y, P_(H)−(1−1/y)·MA or (P_(H)+P_(L))/x when bothP_(L) and P_(H) are valid is in an embodiment equivalent. Accordingly,the phrase “setting the threshold to a fraction of MA optionally plus abias” encompasses setting the threshold according to (P_(H)+P_(L))/x. Inan embodiment x is arranged so that P_(L)<(P_(H)+P_(L))/x<P_(H). In anembodiment x and/or y is determined so that the threshold is equal tothe average obtained from a DC balanced signal content. In an embodimentthe method comprises setting the threshold determined in the manner ofthe prior art receivers such as those cited above. In an embodimentsetting phrase “setting the threshold to a fraction of MA optionallyplus a bias” encompasses setting the threshold based on an average (AVG)of the electrical signal. In one such embodiment the binary content ofthe binary optical signal is encoded to ensure DC balance except forlong series of CIDs. Accordingly, the average of the signal may providea suitable threshold.

In an embodiment at least one of the values P_(L) and P_(H) is obtainedvia at least one peak detector. In an embodiment the receiver circuitoff-sets the electrical signal for example by a threshold value whichseeks to be centered between the values P_(L) and P_(H). In anembodiment this off-set also off-sets the value detected by the peakdetector and which therefore measures an off-set version of the high orlow value of the optical signal. One example of such an implementationof peak detectors may be found in FIG. 6 of U.S. Pat. No. 5,371,763. Inan embodiment the off-set value is a previous determined value, such asthe previously set threshold. In an embodiment the present values P_(L)and P_(H) are therefore obtained relative to a previous determinedthreshold value. In one such example the receiver circuit shunts avariable current from the photocurrent from the photo detector as a wayto set the threshold. Accordingly, P_(L) and P_(H) may depend on thecurrent setting of the threshold for example when measured as the peakvalues from a voltage across a load resistors through which theremaining photocurrent runs. In another embodiment the peak detectorsare arranged to process a signal which is substantially proportional tolight impinging on the photo detector and therefore the values P_(L) andP_(H) are determined as values proportional to a current from the photodetector in response to the optical signal.

As will be well known to the skilled person, peak detectors may beconstructed in several ways. In an embodiment the peak detector willhave a limited bandwidth BW_(P) and/or the peak detector will bepreceded by a low pass filter resulting in an effective bandwidth BW_(P)for the peak detector including the filter. In an embodiment this allowsthe peak detector to ignore undesirable variations of the signal such asnoise components and/or peaks caused by imperfections in the transmittersuch as overshoot or undershoot. In an embodiment the peak detectorcomprises an amplifier, such as a difference amplifier, with a bandwidthBW_(P). In an embodiment there is a trade-off between power consumptionof such an amplifier and extent of its bandwidth to high frequencies. Itmay therefore be beneficial to design the peak detector with a lowerbandwidth. The same may be true for a transmitter where a light sourceis driven to provide the specified bandwidth. If the transmitter isbandwidth limited, the rectangular shape of single 1 bit may be reducedto a soft peak which only achieves the peak value for a small durationof the bit period. In the event of consecutive transmission of the samevalue even a bandwidth limited transmitter will, in an embodiment,transmit the peak value for a substantial part of the time. In anembodiment where the receiver is paired with such an embodiment of atransmitter, it may be advantageous to allow the peak detector to have abandwidth with a cut off frequency below the required frequency fordetecting the peak value of a single bit.

In an embodiment the peak detector has an effective bandwidth, the peakdetector comprises a difference amplifier with a bandwidth BW_(P) withan upper cut off frequency corresponding to the time period of one halfbit or more, such as 1 bit or more, such as 2 bits or more, such asequal to 3 bits or more, such as equal to 4 bits or more, such as equalto 5 bits or more, such as equal to 10 bits or more, such as equal to 20bits or more. In an embodiment the peak detector comprises a hold and/orreset function. In an embodiment the peak detector requires a number ofconsecutively identical bits to validly detect a peak value. In one suchembodiment the peak detector holds this detected value until a newstring of consecutively identical bits is received. In an embodiment thedetermination of ‘when to hold and/or reset’ is incorporated into thepeak detector. In an embodiment this determination is incorporated intothe TAC discussed below. In an embodiment holding the value of the peakdetector occurs within the TAC. TAC (threshold adjustment circuit) is acircuit or combination of circuits that provides automatic orsemi-automatic adjustment of the threshold during operation such asfeedback loops and controllers.

In an embodiment the peak detector incorporates a capacitor or anequivalent element which is charged to the measured peak value. In anembodiment the peak detector therefore has a characteristic dischargetime after which the output value changes significantly in the event oflack of input, in an embodiment this corresponds to the time span of 1bit or more, such as 2 bits or more, such as equal to 3 bits or more,such as equal to 4 bits or more, such as equal to 5 bits or more, suchas equal to 10 bits or more, such as equal to 20 bits or more, such asequal to 30 bits or more, such as equal to 40 bits or more, such asequal to 50 bits or more, such as equal to 100 bits or more, such asequal to 10³ bits or more, such as equal to 10⁴ bits or more. In anembodiment the time span is equal to 10⁴ bits or shorter, such as equalto 10³ bits or shorter, such as equal to 10² bits or shorter, such asequal to 10 bits or shorter, such as equal to 5 bits or shorter, such asequal to 4 bits or shorter, such as equal to 3 bits or shorter, such asequal to 2 bits or shorter, such as equal to 1 bit or shorter.

The criterion for validity P_(H), P_(L) and/or MA may depend on thedesign of the receiver and/or the use of P_(H), P_(L) and/or MA. Forexample, a receiver where the threshold is determined by (P_(H)−P_(L))/xvalidity of P_(H) and P_(L) values often requires that the measuredvalue substantially reflect the true value. Therefore in an embodimentvalidity P_(H), P_(L) and/or MA require that the value reflects the truevalue within a certain accuracy, such as e.g. within 30%, such as e.g.within 20%, such as e.g. within 15%, such as e.g. within 10%, such ase.g. within 5%, such as e.g. within 1%, such as e.g. within 0.5%.

In co-pending U.S. patent application 61/506,842 (which is herebyincorporated in its entirety) embodiments are provided where peakdetection of the electrical signal is referenced against peak detectionof the signal of a reference stage. This method cancels out patterndependence in the peak detectors at least in so far that the peakdetectors have substantially the same pattern dependence for example bybeing substantially identical. An example of this method is provided inthe examples of the present text where a limiting amplifier correspondsto the reference stage. While this method allows for a greatervariability in the binary content compared to most prior arts systemswhich require a substantially balanced content, this method may not beeffective for all signal content. Accordingly, the requirement on thebinary content so that P_(H), P_(L) and/or MA are valid may in anembodiment be relaxed. Examples of binary content where P_(H), P_(L)and/or MA are invalid may include a long series of CIDs in which casethe peak detector arranged to measure the other logic value to drift soe.g. P_(H) and P_(L) outputs the same or indistinguishing value which inturn may make it impossible to set a suitable threshold based on thismethod alone. For embodiments which rely on such embodiments of U.S.patent application 61/506,842, invalidity of P_(H), P_(L) and/or MA maybe taken to mean that the value(s) is not reliable even though patterndependency at least to some extent cancels out. Depending on the designof the peak detector a single or few bits of one logic value during along series of another value may be insufficient to provide a reliabledetection of the one logic value. Accordingly, in an embodiment thelength of a series of CIDs of logic-high or logic-low is determineddisregarding series of CIDs of the other logic value shorter than aminimum length, such as shorter than or equal to 10 bits, such asshorter than or equal to 5 bits, such as shorter than or equal to 3bits, such as shorter than or equal to 2 bits, such as shorter than orequal to 1 bit.

In an embodiment the method comprises initializing the receiver forexample to establish a modulation index of the transmitter with thereceiver. In an embodiment the initialization is performed duringreception of a known signal. In an embodiment, the optical receiverforms part of a two-way link having a far end receiver, a near endreceiver, a far end transmitter and a near end transmitter. In anembodiment the near end receiver and transmitter cooperate todetermining whether an optical connection (typically via a fiber or air)is established to a far end. In an embodiment the transmitter transmitstest pulses at regular intervals. These pulses are in an embodiment keptat a relatively high power level to ensure reception at the far end evenif the link is poor. On the other hand, the pulses are in an embodimentkept short to ensure eye safety. At the same time the receiver isarranged to detect test pulses from the far end. In an embodiment thereceiver circuit is arranged to have a specific mode of operation onlysuitable for detection of an incoming pulse. In this way the receivercircuit may reduce power consumption during time where no connection isestablished. In the event that a test pulse is received, the near endtransmitter may begin to transmit one or more preset data seriesallowing the far end receiver to initialize. Similarly, the far endtransmitter may begin to transmit one or more preset data seriesallowing the near end receiver to initialize. In an embodiment, thisinitialization process further comprises transmitting data qualityparameters between the near end and far end, e.g. to optimize thetransmitters. In an embodiment it is advantageous that the transmittertransmits with minimal power consumption while at the same timeproviding robust reception of the signal at the other end. The aboveconsideration regarding initialization and self-configuration of anoptical link is the focus of co-pending U.S. Provisional Application fora Patent 61/228,848.

In an embodiment the method further comprises determining an average(AVG) of the electrical signal, IES. In an embodiment the circuitcomprises an averaging circuit (such as a suitable low pass filter)whereas in an embodiment AVG is determined as part of the TAC. The valueof AVG may be applied to determine the DC balance of the signal i.e. theratio of ZEROs and ONEs. As will be discussed further below in regard tovalidation of the P_(L), P_(H) and MA values, an average substantiallyoffset from the threshold may in an embodiment indicate that the high orlow values are rarely experienced so this peak value may not beaccurately determined (i.e. valid).

In an embodiment the AVG is calculated from a time period, such as equalto or longer than 10⁻⁹ seconds, such as equal to or longer than 10⁻⁸seconds, such as equal to or longer than 10⁻⁷ seconds, such as equal toor longer than 10⁻⁸ seconds, such as equal to or longer than 10⁻⁵seconds, such as equal to or longer than 10⁻⁴ seconds, such as equal toor longer than 10⁻³ seconds, such as equal to or longer than 10⁻²seconds, such as equal to or longer than 10⁻¹ seconds, such as equal toor longer than 1 second. In an embodiment the average AVG is taken overa time period, such as equal to or shorter than 1 second, such as equalto or shorter than 10⁻¹ seconds, such as equal to or shorter than 10⁻²seconds, such as equal to or shorter than 10⁻³ seconds, such as equal toor shorter than 10⁻⁴ seconds, such as equal to or shorter than 10⁻⁵seconds, such as equal to or shorter than 10⁻⁸ seconds, such as equal toor shorter than 10⁻⁷ seconds, such as equal to or shorter than 10⁻⁸seconds, such as equal to or shorter than 10⁻⁹ seconds. In an embodimentthe average is over a period substantially equal to 1 bit or more, suchas equal to 2 bits or more, such as equal to 3 bits or more, such asequal to 4 bits or more, such as equal to 5 bits or more, such as equalto 10 bits or more, such as equal to 20 bits or more, such as equal to30 bits or more, such as equal to 40 bits or more, such as equal to 50bits or more, such as equal to 100 bits or more, such as equal to 10³bits or more, such as equal to 10⁴ bits or more, where the duration of 1bit may be determined from the bit rate BR. In an embodiment the timeperiod is substantially equal to 10⁴ bit or less, such as equal to 10³bits or less, such as equal to 100 bits or less, such as equal to 50bits or less, such as equal to 40 bits or less, such as equal to 20 bitsor less, such as equal to 10 bits or less, such as equal to 5 bits orless, such as equal to 3 bits or less, such as equal to 2 bits or less,such as equal to 1 bit or less.

In an embodiment the AVG is obtained from a period related to thedischarge time of the peak detectors discussed above. In an embodimentAVG is calculated from a time period corresponding to 10% or more of thedischarge time, such as 20% or more of the discharge time, such as 30%or more of the discharge time, such as 40% or more of the dischargetime, such as 50% or more of the discharge time, such as 60% or more ofthe discharge time, such as 70% or more of the discharge time, such as80% or more of the discharge time, such as 90% or more of the dischargetime, such as 100% or more of the discharge time, such as 110% or moreof the discharge time.

In an embodiment the method further comprises applying an activitydetector (i.e. a circuit with a function) to indicate the activity inthe electrical signal. As will be explained below, an activity detectormay in an embodiment provide the TAC with an indicator of the propertiesof the signal second. Depending on the construction of the peak detectorthe value determined by the peak detector may depend on the peak (i.e.the size and shape of the signal corresponding to a bit) and the bitpattern i.e. the ratio of ZEROs to ONEs and/or how often CIDs arereceived consecutively e.g. for two, three, four, five or ten bits in arow. To determining the validity of the output from a peak detector itmay therefore be useful to apply an activity detector. In an embodimentthe activity detector is a circuit that either provides an indicatorvalue or allows the TAC or other circuitry to calculate an indicatorvalue for one or more of:

-   -   1. Number of transitions between (logic) high and (logic) low        within a predetermined time period.    -   2. Number incidences of high and/or low values longer than a        predetermined length over a predetermined time period, in other        words the number of CIDs of logic high and/or low values longer        than a predetermined length (e.g. 3) over a predetermined time        period.

In an embodiment the predetermined time period may be any of the valuescited in relation to the determination of the average above and/or inregard to present and previous above. In an embodiment the predeterminedtime period is not specifically determined but rather a time within asuitable range is used. In an embodiment the indicator value is avoltage or current proportional to the number. In an embodiment theindicator is a digital value proportional to the number. In anembodiment the indicator is a binary value which indicates whether thenumber exceeds a specified threshold, such as 1 or more, such as 2 ormore, such as 10 or more, such as 100 or more. In an embodiment theindicator values are updated similarly as a rolling average i.e.substantially continuously whereas in an embodiment the activitydetector provides indicator(s) for non-overlapping time segments. Thelatter may in an embodiment be implemented via a counter counting, thenumber of relevant events and the counter having a hold register forholding a value and a reset function. Storing of the value in the holdregister and resetting the counter may then, for example, be controlledby a digital oscillator with a set frequency. In this way the indicatorvalue may for example be updated e.g. each time the predetermined timeinterval expires.

In an embodiment two or more peak detectors are applied to detect thehigh value or equivalently different functionality may be applied by thesame peak detector. Each of the two or more peak detectors has abandwidth and discharge time so that it is suited to detect the peakvalue for a specific activity level. In an embodiment one peak detectoris designed to obtain the peak value for a very long constant signal,whereas a second peak detector is suitable for detecting a peak valuefor a medium amount of signal activity. In an embodiment an indicatorfrom the activity detector allows the TAC to determine which peak valueto apply. In an embodiment applying separate peak detectors and/orapplying different functionality in one peak detector depending onsignal activity may provide better noise resistance as the same noisewill influence determination of the peak differently depending onwhether the signal changes frequently (i.e. have many transitions) or issubstantially constant. In an embodiment the bandwidth for the two ormore peak detectors is different where the upper bandwidth is lower forthe peak detector arranged to detect the peak value of a substantiallyconstant signal.

A threshold adjustment circuit (TAC) may in an embodiment be implementedin digital circuitry, analog circuitry or a combination thereof. It mayin an embodiment be a separate circuit, either integrated along with theremaining receiver circuit or alone. In an embodiment the TAC in wholeor in part is considered as an integrated controller which may beprogrammable. In an embodiment a part of the TAC belongs to an externalcircuit such as in an external controller. For example the determinationof validity of the peak values as well as the determination of thresholdmay reside with an external controller not integrated with theelectrical high speed data path of the receiver, e.g. transimpedanceamplifier and a decision circuit, whereas circuitry for adjusting thethreshold and/or circuitry assisting in applying the peak values (e.g.the circuit of FIG. 4 below) could be internal circuits while still partof the TAC. The term ‘decision circuit’ refers to a circuit arranged toprovide an essentially binary data stream from an analog input signal.Other terms used in the art are comparator and digital quantizer.Commonly a limiting amplifier is applied as the decision circuit.

In an embodiment one or more functions belonging to the TAC areintegrated into one or more functions of the receiver circuit. Oneexample could be integrating one or more functions of the TAC into oneor more peak detectors so that each peak detector may determine whetherthe apparent value detected from the signal should be deemed valid. Inthe present text it is assumed that the validation of the values P_(H),P_(L) and/or MA resides in a distinct TAC circuit but it should be keptin mind that the invention is not limited to such implementation.

Setting of the threshold may be confined to a separate circuit ordistributed in the receiver circuit such as into the peak detectors. Thethreshold may be set in one or more of several parts of the receivercircuit depending on the design of the circuit. In an embodiment thethreshold is set by a shunt current source suitable for shunting part ofthe current from the photo detector so as to adjust signal input to adecision circuit relative to a reference value for the decision circuiteffectively adjusting the threshold for the binary electrical signal. Inan embodiment the threshold is set by an offset for an amplifier, suchas an amplifier converting a current from the photo detector to avoltage. Such an amplifier could for example be a transimpedanceamplifier or an equivalent. In an embodiment the threshold is set as athreshold for a decision circuit, such as a differential amplifierhaving the threshold as one input and an electrical signal as the other.This amplifier may in an embodiment be wholly or partly limiting.

In an embodiment characteristics of IES are measured at different nodesin the circuit e.g. for threshold, average and peak detection. Forsimplicity the present text considers the corresponding value of IES atthe same node, see for example FIG. 1 before and after the differentialamplifier 103.

As discussed above, the determination of the validity of either P_(H) orP_(L) resides with the TAC either as a separate circuit or integratedinto one or more functions of the remaining receiver circuit. In anembodiment the present P_(H) value and/or MA is deemed invalid in theevent that the current signal over an extensive time period adjacent todetermining the present P_(H) value has been dominated by low values,i.e. no or few high values have been received for an extensive timeperiod. Similarly, the present P_(L) value and/or MA is invalid in theevent that the current signal over an extensive time period adjacent todetermining the present P_(L) value has been dominated by high values.In an embodiment “dominated” is more than 50% of the time, such as morethan 60%, such as more than 70%, such as more than 80%, such as morethan 90%, such as more than 95%, such as 100%.

In an embodiment dominated by for example logic-high values refers to noor few occurrences of 2 or more identical low value bits consecutivelyin a period of time, such as 3 or more bits, such as 4 or more bits,such as 5 or more bits, such as 6 or more bits, such as 10 or more bitsand vice versa. In other words, this means that the optical signal maybe dominated by high values when a specified length of CIDs of lowvalues occurs rarely or not at all. In an embodiment, the term rarelyrelates to the properties of the respective peak detector. The peakdetector may require a number of occurrences of CIDs of specified lengthto obtain an accurate measurement. Rarely refers here to a number belowthis value. In an embodiment rarely refers to less than 1000, such asless than 100, such as less than 50, such as less than 10, such as lessthan 5, such as less than 2 within the time period.

In an embodiment the extensive time period takes one of the possiblevalues specified in relation to the determination of AVG, including thedischarge time of the peak detectors or a period specified in relationto the span between previous and present values.

In an embodiment the threshold (TH_(P)) is compared to the average AVGof the electrical signal to determine the validity of P_(L), P_(H)and/or MA. In an embodiment a threshold TH_(P) determined using eitherthe P_(L) or P_(H) as bias and a fraction of MA is compared to AVG todetermine the validity of P_(L), P_(H) and/or MA. For a DC balancedsignal AVG and TH_(P) should preferably be substantially equal. In anembodiment TH_(P) and AVG are obtained at different times. For example;in an embodiment TH_(P) is a previous TH_(P) whereas AVG is the presentAVG. One such previous value could in an embodiment be the latest valueof TH_(P) and this threshold is assumed substantially constant when theDC balance of the signal is evaluated. In an embodiment the time spanbetween obtaining TH_(P) and AVG is substantially less than the timeperiod within which the transmitter may be expected to drift or opticalloss change, i.e. the previous threshold TH_(P) should still be a validthreshold at the time of comparison. In an embodiment this time span maybe any of the times or in relation to the previous and present values orAVG.

In an embodiment a substantial deviation between AVG and TH_(P)indicates that either present P_(L) or P_(H) is invalid, so that

-   -   a. when AVG>TH_(P) then P_(L) is deemed invalid and    -   b. when AVG<TH_(P) then P_(H) is deemed invalid.

In an embodiment the deviation is calculated as

${100{\% \cdot {{2\frac{{AVG} - {TH}_{P}}{P_{H} - P_{L}}}}}},$so that an AVG equal to either the previous P_(L) or P_(H) is a 100%deviation. In an embodiment a substantial deviation is larger than orequal to 5%, such as larger than or equal to 10%, such as larger than orequal to 20%, such as larger than or equal to 30%, such as larger thanor equal to 40%, such as larger than or equal to 50%, such as largerthan or equal to 60%, such as larger than or equal to 70%, such aslarger than or equal to 80%, such as larger than or equal to 90%, suchas larger than or equal to 95%.

As previously mentioned the peak detectors commonly have a dischargerate. Therefore, the decrease of the output of a peak detector with asimilar rate may be an indicator that respective signal value is notpresent and the measurement may therefore be invalid. Therefore in anembodiment a rate of change in P_(L) or P_(H) indicates that the P_(L)or P_(H) is invalid. In an embodiment the rate of change, of e.g. AVG,in the event of a long set of CIDs may be different from the dischargerate of the peak detector. Therefore, their relative change may be anindicator of the signal comprising e.g. a long set of CIDs. Furthermore,in this event the other peak detector will likely exhibit little changewhich may be a further indicator of a long set of CIDs. Accordingly, inan embodiment a change in P_(L) or P_(H), optionally adjusted for anoffset relative to the binary electrical signal, relative to the othervalue and/or an average value related to the electrical binary signalindicates that the changing P_(L) or P_(H) is invalid.

In an embodiment one or more of the above indicators may be combined todetermine the validity of the measurement of P_(L) and/or P_(H). Forexample the activity detector may be applied in combination withdeviation of AVG and TH_(P) to determine validity. This is furtherdiscussed in relation to the activity states below.

In an embodiment one or more of the indicators and values available tothe TAC such as indicators from the activity detector optionally incombination with the AVG, a previous threshold, a present threshold, aprevious P_(L) and/or P_(H) and/or present P_(L) or P_(H), a rate ofchance in the P_(L) or P_(H), a relative change in the P_(L) and/orP_(H), a deviation between AVG and a threshold, one or more indicatorvalues obtained via the activity detector, and the domination of theother value in determining P_(L) or P_(H), allow the TAC to determine astate of the signal. In an embodiment the state of the signal is takento mean that the TAC considers the factors for setting the state whendetermining the validity of the peak measurement. In an embodiment theTAC applies an indicator to indicate the state but this may not benecessary. Therefore, in an embodiment the present P_(L) or P_(H) valueis determined by a TAC from two or more of the parameters mentionedabove.

In an embodiment the states comprise two or more of the followingpossibilities:

-   -   High activity    -   Medium activity    -   Low activity

In an embodiment “high activity” is taken to mean that the signalcomprises many transitions, such as substantially like a dottingsequence. As discussed above, a peak detector may in an embodiment nothave sufficient bandwidth to determine P_(L) or P_(H) from a series ofCIDs shorter than e.g. 3 bits. In an embodiment ‘high activity’ isdefined as sufficiently high activity so that substantially accuratedetection of either P_(L) or P_(H) is unlikely. In an embodiment“unlikely” is less than 99%, such as less than 95%, such as less than90%, such as less than 75%, such as less than 50%. In anembodiment“substantially accurate” is taken to mean within 50%, such aswithin 30%, such as within 15%, such as within 10%, such as within 5%,such as within 1%. In an embodiment the limited bandwidth in thedetermination of P_(L) or P_(H) will cause these values to beunderestimated. In an embodiment this error may be estimated from theactivity and/or the DC balance of the signal and therefore acompensation for this error may be possible. Therefore, in an embodimentthe method comprises estimating the error and compensating the peakvalues accordingly before updating the threshold.

As discussed above, in an embodiment the receiver references the peakdetection of the electrical signal (i.e. P_(L) and P_(H)) against peakdetection of the signal from a reference stage to cancel the patterndependency of the peak detectors. In an embodiment, this method allowssetting of a suitable threshold even for binary content exhibiting highactivity.

In an embodiment the AVG value may be applied in determining whether theactivity is high in the sense that accurate detection is unlikely. Inthe event that AVG deviates substantially from the threshold it may belikely that a substantial amount of consecutively high or low values arefound in the signal pattern and therefore the corresponding peak valuemay still be detected. In an embodiment such a situation will cause theTAC to determine that the state of the signal is “medium activity”instead of “high activity” even though a high number of transitions mayoccur. In an embodiment the activity detector provides an indicatorrelating to the number of consecutively identical bits. In one suchembodiment the comparison of AVG with the threshold may be less relevantin order to determine whether P_(L) or P_(H) may be determined robustly.Instead the TAC may in an embodiment deem that the signal comprises“high activity” only when this is not the case.

One or more actions may be taken by the TAC when a high activity stateis determined. In an embodiment a state of high activity causes thereceiver to perform at least one of the following:

-   -   Setting the threshold based on AVG of the signal.    -   Setting the threshold based on P_(L) and P_(H) values relative        to corresponding P_(L) and P_(H) values measured from the output        of a reference stage, such as a limiting amplifier.    -   Determining the DC balance of the signal by determining a        deviation between AVG and TH_(P), and adding suitable offset        values to P_(L) and/or P_(H) optionally depending on the        deviation.    -   Maintaining a constant threshold.

In an embodiment a setting of “Medium activity” corresponds to theoccurrence of sufficient activity of transitions so that both P_(L) andP_(H) are likely attainable while at the same time sufficiently longstrings of consecutively identical values occur relative to thecharacteristic times of the peak detectors, so that both P_(L) and P_(H)are likely attainable with sufficient accuracy. In an embodiment asetting of medium activity indicates that P_(L) and P_(H) are valid.

In an embodiment low activity indicates that few transitions occurswithin a time frame where the transmitter is likely to drift or opticalloss change or a time frame corresponding to the discharge time of thepeak detectors so that their output is not reliable. In an embodimentlow activity corresponds to long series of CIDs mentioned above. In thissetting the receiver may in an embodiment utilize a specialized peakdetector with a long discharge time or a reset and hold as mentionedpreviously. However, in an embodiment low activity corresponds to P_(L)or P_(H) deemed invalid and the threshold is set accordingly based onthe toggling amplitude.

In an embodiment the threshold is updated at certain intervals which arepreferably shorter than the time span in which the transmitter isexpected to drift or changes in insertion loss are likely to occur.There may therefore be a significant difference for links applied ine.g. a computer server which is rarely physically moved and a linkapplied to connect two consumer products where an optical fiber may beexposed to bending. The time frame may therefore in an embodiment be anyof times cited in relation to the span between previous and presentvalues. Drift in the transmitter may for example be caused bytemperature which, in an embodiment, is compensated to some degree bythe driver of the light source. Accordingly, large drift may in anembodiment be expected only for large temperature changes. Anothercontribution to drift may be aging depending on age, time in operationand/or amount of optical energy generated by the light source. Dependingon the light source such aging may cause more or less abrupt changes. Askilled person may therefore, in an embodiment, take these times intoaccount so that the threshold is updated sufficiently often toaccommodate changes while not so often that an intolerable increase inpower consumption occurs in the receiver.

As should now be clear to the skilled person, the design of the peakdetectors (or any other circuit for determining the logic-high and/lowvalue of the signal), the design of the receiver (e.g. whether areference stage is applied to cancel pattern dependence) and theexpected binary signal content must in an embodiment be balanced toensure correct setting of the threshold in the receiver. In embodimentswhere it is needed the designer must device suitable tests for detectingwhen a peak value may not be valid. The previous text has providedseveral examples of such tests but other indicators may be devisedwithout parting from the scope of the invention. It may in an embodimentbe necessary for the designer of the receiver to balance the complexityof the indicators with the robustness of the system. Often more complextests will lead to a more complex circuit which potentially has highpower consumption and/or chip area consumption. A designer could, forexample, apply a simulation tool to determine whether a specified designof peak detectors (or another method of determining the high and/lowvalue) and the designed indicators and TAC are sufficiently robust.

If there are any inconsistencies between text incorporated by referenceand the text explicit provided herein, the text explicit provided hereinshall prevail.

DESCRIPTION OF DRAWINGS AND EXAMPLES

The invention will be explained more fully below in connection with apreferred embodiment and with reference to the drawings in which:

FIG. 1 is a schematic illustration of a receiver according to theinvention in a mode where the binary content of the signal is not a longseries of CIDs.

FIG. 2 shows the receiver according to FIG. 1 in a mode suitable for along series of CIDs.

FIG. 3 shows a simulated signal electrical input signal to the receiverof FIGS. 1 & 2.

FIG. 4 shows a close up of the signal shown in FIG. 3.

FIG. 5 shows the output of the non-limiting amplifier 103 of the signalof FIG. 3

FIG. 6 shows a close up of the signal of FIG. 5.

FIG. 7 is a schematic illustration of a transmitter.

FIG. 8 is a schematic illustration of a receiver according to theinvention where the receiver can operate in a training mode and in anoperating mode suitable for a long series of CIDs.

The figures are schematic and may be simplified for clarity. Throughout,the same reference numerals are used for identical or correspondingparts.

FIG. 1 shows a receiver circuit of the invention in a mode where thebinary content of the signal is sufficiently active that the peakdetectors 109-112, 114-117 provide sufficient information of thelogic-levels to reliably set a decision threshold. FIG. 2 shows the samereceiver in a mode where the binary content comprises a long series ofCIDs. As discussed an activity detector, optionally in combination withthe TAC, may be used to determine when the receiver optimally operatesin the mode of FIG. 1 or of FIG. 2. The activity detector is not shownand the TAC is partially shown as 118, 119, 202-204, but circuitrycontrolling the mode of the receiver is not shown. This could beperformed by an internal or external digital controller or constructedusing analog circuitry. The receiver circuit 1 is connected to a photodiode 101 for receiving the binary optical signal and converting it intoa current which is converted to voltage V_(TIA) via the transimpedanceamplifier 102. Here V_(TIA) may be seen as the intermediate electricalsignal (IES) discussed above, which in this embodiment is input to thedifferential amplifier 103 and the decision threshold V_(REF) is theother input. In this embodiment the differential amplifier 103 is alinear amplifier (i.e. non-limiting) so the toggling signal isdetectable in the output, IES_(DIFF), of the amplifier 103. The gains of103 are in this description considered to be 1 for simplicity but maytake any suitable value. The outputs of the linear differentialamplifier 103 are connected to diodes 105 a and 105 b connected to theinverted and non-inverted output, respectively. Their collective outputprovides the maximum value from the inverted and non-inverted outputwhich is logic-high from the non-inverted output and logic-low from theinverted output. For a threshold value, V_(REF), where the output of theamplifier is symmetric about V_(REF), the collective output of thediodes 105 will in an embodiment provide a substantially constant outputrelated to V_(REF) plus half the modulation amplitude. This output willexhibit the toggling signal with about half the toggling amplitude. Thediodes 105 may be said to form a so-called full wave rectifier circuitwhich may include the differential amplifier 103 depending ondefinition.

The toggling amplitude is measured by the toggling amplitude detectorcircuit 106 where two peak detectors linked to the output of the diodes105 provide input to a differential amplifier. The measured togglingamplitude is compared to the reference (AmpRef) 108 by the amplifier 107which is linked to a gain control of the transimpedance amplifier 102.In this way a feedback loop is formed arranged to maintain constanttoggling amplitude. For example, in the event of increased loss betweentransmitter and receiver due to bending of a fiber, the gain of thetransimpedance amplifier 102 will be increased to achieve constanttoggling amplitude. If the modulation index from the receiver isconstant the modulation amplitude of the IES will hence also bemaintained substantial constant.

The non-inverted output of the amplifier 103 is further connected topeak detectors 109 and 110 arranged to measure values related to thelogic-high (V_(PH,LINP)) and logic-low (V_(PL,LINP)) values,respectively. Similarly the inverted output is connected to peakdetectors 111,112 arranged to measure values related to the logic-high(V_(PH,LINQ)) and logic-low (V_(PL,LINQ)) values of the inverted signal,respectively. The peak detectors 109-112,114-117 and of those 106 areshown here in schematic form as a diode connected to a capacitor but mayin principle have any suitable design for suitable peak detection. Theoutput of the amplifier 103 is further connected to limiting amplifier113 which serves the function of reference stage in the terminology ofthe methods disclosed in co-pending U.S. provisional application61/506,842. The output swing of amplifier 113 is adjustablecorresponding to adjusting the modulation current of a reference stage.The output of the amplifier 113, IES_(LIM), is connected to the peakdetectors 114-117 respectively providing values V_(PL,LIMP),V_(PH,LIMP), V_(PL,LIMQ) and V_(PH,LIMQ) which mirror peak detectors109-112. In the present embodiment the V_(PL,LIMP), V_(PH,LIMP),V_(PL,LIMQ) and V_(PH,LIMQ) values are arranged as input in feedbackloops implemented by the comparator setup 118 and the up/down counters119 and 120. The two feedback loops are arranged to adjust the thresholdV_(REF) and the output swing of the amplifier 113 so that the differencein logic-high of the non-inverted signal on either side of the amplifier113 is substantially equal to the difference in logic-high of theinverted signal. By way of the limiting amplifier 113 and thereferencing of the output of the peak detectors on either side of thisamplifier the pattern dependency of the peak detectors will at least tosome extent cancel out. In an embodiment other signal parameters areapplied instead of one or more of the peak detectors shown here as longas the degrees of freedom in the signal can be resolved. In anembodiment two signal parameters from each side of the reference stageare sufficient such as two values selected from average, modulationamplitude, P_(H), P_(L).

It is noted that this embodiment does not necessarily measure themodulation index as it may be sufficient to set AmpRef to a level wheresecond signals will have an IES in the linear regime of the amplifier103. However, in an embodiment the receiver comprises a controller foradjusting AmpRef for example using the adjustment by the counter 119 asinput as the setting of the amplifier 113 by the feedback loop providesan indicator of the modulation amplitude of the signal. The peakdetector outputs (e.g. 109,110) may also be applied but, depending onthe design, the peak detectors may introduce pattern dependency in thedetermination of the modulation amplitude.

In an embodiment the peak detectors 109-112, 114-117 have a bandwidth sothat they average over the toggling signal. This may provide for a morestable receiver with relatively little activity in the feedback loopsfor constant signal characteristics. In an embodiment the peak detectors109-112, 114-117 have a bandwidth so that they follow the togglingsignal. This may in an embodiment provide for a receiver which respondsfaster to changes in the optical signal.

The receiver further comprises circuitry for regenerating the binarycontent. In an embodiment the output of the limiting amplifier 13 isapplied for that purpose followed by signal conditioning and an outputdriver for relaying the binary content as an electrical binary signal.

FIG. 2 shows the receiver of FIG. 1 in a mode suitable for receiving asignal that is a long series of CIDs or sufficiently dominated by onelogic value that the other value is not sufficiently well detected bythe respective peak detectors to set a reliable threshold. As anexample, the following discussion assumes that a long series oflogic-low is received. The connection 201 connecting a clock signal tothe up/down counter 119 is interrupted in this mode resulting in aconstant output swing of the limiting amplifier 113. The togglingamplitude is detectable via the diode 105 b connected to the invertedoutput of the amplifier 103. As substantially only logic-low is receivedonly peak detectors 109,112,114 and 117 will likely provide a usefulsignal and the output of the limiting amplifier 113 will be constantLIMP and LIMQ on the non-inverted and inverted outputs, respectively.The inverted output will be at logic-high, so that the CID sign detector202 will set the switch 3 so the values from peak detectors 109,112,114and 117 are used to adjust the threshold V_(REF). The output from thepeak detectors is compared so that the up/down counter 120 adjusts thethreshold so that the modulation amplitude out of the amplifier 103equals that of 113. In the event that characteristics of the incomingsignal changes corresponding changes in the toggling amplitude willensure adjustment of the gain of the transimpedance amplifier via thefull-wave rectifier 103,105 and the circuits 106,107,108. A change inoffset relative to V_(REF) will be adjusted by the up/down counter 120,so a viable threshold is maintained even when receiving a long series ofCIDs.

In an embodiment the up/down counter 119 is not disabled but insteadconnected to the same comparator 204 or 205 as the up/down counter 120.This may for example be preferable for a receiver where the peakdetectors 109-112, 114-117 are fast enough to resolve the togglingsignal.

FIGS. 3-6 show results of simulations of the receiver of FIGS. 1 and 2.The units on the y-axis are arbitrary and relate to specificities of thesimulation. FIG. 3 shows an simulated input signal (i.e. output of thephotodiode 101) comprising sections 31, 33, 35, 37 of active binarysignal (single bits are indistinguishable) and long series of CIDs 32,34, 36 and 38. During periods 32-35 the modulation amplitude andlogic-low is changed. During periods of CIDs (32,34,36,38) the togglingsignal is clearly identified as a broadening of the signal. FIG. 4 showsa subsection of the input signal where sections 41 and 42 correspond toparts of the sections 31 and 32. In section 41 the modulation amplitudetoggles between about 130 units and about 150 units symmetrically aboutY0=140 i.e. with average modulation amplitude of about 140 units. Thetoggling signal is identifiable as binary signal with amplitude of about10 units which modulates the modulation amplitude, so the modulationindex is in this case 10/140≈7%. Section 42 shows how the logic lowvalue is modulated by the substantially binary toggling signal withamplitude of about 5 units.

FIG. 5 shows the output from the non-limiting amplifier 103. During thefirst 2 ms of section 51 the feedback loops settle and the receiverinitialization may be said to be complete. The threshold settlesrelatively quickly close to zero whereas the modulation amplitudesettles at a slower rate. It is notable that the regenerated signal issubstantially constant through the periods 52-55 corresponding to theperiods 32-35 where the characteristics of the input signal change.Accordingly, the example demonstrates the capability of the receiverincorporating the invention to set a suitable threshold even during longperiods of CIDs and to compensate for changes in the input signal.

FIG. 7 shows an optical transmitter driver circuit comprising a lightsource 301 and a waveguide 302. The light source-waveguide configurationis known as a Maeh-Zender Modulator. The light source is emitting light.The emitted light is coupled into the waveguide 302 via a coupler 303.In the waveguide 302 the light is separated into a first and a secondbranch 304 a and 304 b. The first branch 304 a is transmitting the lightdirectly through the waveguide 302 to a merging region 305. The secondbranch 304 b is transmitting the light with a modulated delay throughthe waveguide 302 to the merging region 305.

The modulated delay is obtained by subjecting the second branch to apower provided by a modulated voltage via electrodes L1, L2. Theelectrodes L1, L2 are electrically connected to be supplied withrespectively the binary signal (indicated with 307) and the togglingsignal supplied from sub-band modulator 308. Each of the electrodes L1,L2 is arranged such that light transferred in the second branch 304 b isdelayed when voltage is applied to one or both electrodes. The sum ofthe applied voltage determines the delay of the time and thereby theresulting light intensity, which due to the modulating of the voltage ismodulated accordingly.

The electrode L1 is regulated by the binary input signal 307 and theelectrode L2 provides the toggling signal from sub-band modulator 308.

The modulating index can be determined as L1/L2.

The sub-band modulator 308 is electrically connected to receive thebinary signal 307 as input via inverter 309 inverting the signal ornon-inverter 310 not inverting signal. The inverter and non-inverter canindependently of each other e.g. simultaneously amplify or limit therespective signal. The sub-band modulator 308 regulates the togglingsignal by switching between applying the inverted signal and applyingthe non-inverted signal.

In the merging region 305 the light from the first and the second branch304 a and 304 b is merged to obtain the modulated binary optical signal.Due to the delay of the light in the second branch 304 b, the delayedlight will be out of phase with light in one or more other brancheswhere the light has not been delayed or where it has bee delayed to adifferent degree. The delayed light will therefore when merged withlight from one or more other branches, therefore more or less extinguishthe light, and since the light is delayed in a modulated fashion, theresulting light will in a similar way be modulated with respect to lightintensity.

The modulated binary optical signal is send further via output coupler306 to be transmitted.

FIG. 8 is a schematic illustration of a receiver according to theinvention which receiver can operate in a training mode and in anoperating mode suitable for a long series of CIDs. The receiver isadvantageously configured to operate in the training mode when thebinary content of the IES signal relatively, and advantageously numberof logic high and logic low is relatively equal. When the binary contentcomprises a long series of CIDs, the receiver advantageously switches tooperating mode

The activity detector is not shown and the TAC is partially shown, butcircuitry controlling the mode of the receiver is not shown.

The receiver circuit is connected to a photo diode 401 for receiving thebinary optical signal and converting it into a current which isconverted to voltage V_(TIA) via the transimpedance amplifier 402. HereV_(TIA) may be seen as the intermediate electrical signal (IES)discussed above, which in this embodiment is input to the differentialamplifier 403 and the decision threshold V_(REF) is the other input. Thedifferential amplifier 403 may be as described in FIGS. 1 and 2. Theoutputs of the differential amplifier 403 are connected to diodes 409 aand 409 b connected to the non-inverted input of the error amplifier410. The inverted input to the error amplifier is a reference voltage413—which may advantageously be adjusted manually. The output of 410 isvia the up/down counter 411 and the DAC 412 connected to the gaincontrol input of the transimpedance amplifier. Thereby this circuitautomatically controls the voltage swing on the output of 403 so thatthe voltage swing is constant and controlled by the reference voltage413.

The differential amplifier 403 is also connected to the error amplifier406 via the resistors 405 a and 405 b. In the training mode the switch417 is connecting the output of 406 to the reference input of theamplifier 403 e.g. via the up/down counter 418 and the DAC 419. In thismode the circuit operates like a conventional DC restoration circuitwith the threshold being the reference input of the differentialamplifier 403.

If the EIS signal has the properties described above the up down counter411 will count up and down as a function of the second togglingamplitude and the difference between the highest count and the lowestcount will be a measure for the toggling amplitude. This difference ismeasured and stored for reference later when the receiver is in anoperating mode suitable for receiving long series of CID.

In the operating mode suitable for receiving long series of CID, theswitch 417 connects the output of the error amplifier 420 with theup/down counter 411. The error amplifier 420 compares the actualmeasured difference between the highest count and the lowest count for411 with the stored value from the training mode and controls theup/down counter 411 accordingly. Thereby the reference to thedifferential amplifier 403 will be equal to the threshold for the signalVTIA—which constitutes the EIS signal even during long periods of CID.

In this way a feedback loop is formed arranged to maintain constanttoggling amplitude. If the modulation index from the receiver isconstant the modulation amplitude of the IES will hence also bemaintained substantial constant.

From the foregoing general discussion of the invention and the exampleof figures it is clear that the invention may be implemented in manyways. The above example should therefore be considered an exampledemonstrating specific implementations of the different components ofthe receiver, e.g. circuit arranged for measuring the second togglingamplitude and circuit for measuring values relating to logic-low andlogic-high in the signal. As will be clear to a skilled person suchimplementations of one function may be combined with other embodimentsof other functions without departing from the scope of the invention.For example, the receiver may in one example be implemented applying thedecision threshold, V_(REF), as one input to a limiting amplifier andV_(TIA) as the other as it is common in many prior arts receivers. Inone such embodiment the limiting amplifiers have a single ended output.Similar to FIG. 1, V_(REF), and the output swing of the limitingamplifier may be adjusted via feedback loops comparing signal parameterson both sides of the limiting amplifier. The receiver further comprisescircuitry for measuring the toggling amplitude, the output of which isarranged to adjust the gain of the transimpedance stage similarly to thereceiver of FIG. 1.

The invention claimed is:
 1. A method for optical transmission of anoptical signal comprising a binary content comprising transmitting theoptical signal with the binary content comprising a first logic-high anda first logic-low defining a first modulation amplitude wherein thefirst modulation amplitude is sub-band modulated with a toggling signalhaving a first toggling amplitude with a first modulation index relativeto said first modulation amplitude, receiving said optical signal withan optical receiver circuit and converting the optical signal to anintermediate electrical signal, IES, said IES having the followingparameters: i. a second logic-high and a second logic-low defining asecond modulation amplitude and ii. a second toggling amplitude having asecond modulation index relative to said second modulation amplitude,providing a decision threshold relative to said IES as a function ofsaid second modulation amplitude, and adjusting the decision thresholdby determine the second toggling amplitude and adjusting said decisionthreshold relative to said IES based on proportionality between thesecond toggling amplitude and said second modulation amplitude.
 2. Themethod of claim 1 wherein said method comprises determine at least oneof the following values a P_(H) value related to the second logic-high,a P_(L) value related to the second logic-low, and a MA value related tothe second modulation amplitude and determining the validity of saidmeasured values.
 3. The method of claim 2 comprising adjusting saiddecision threshold based on one or more of said determined values whensaid determined values are determined to be valid.
 4. The method ofclaim 3 wherein a change in P_(L) or P_(H) relative to the other valueor an average value of the IES indicates that said P_(L) or P_(H) isinvalid.
 5. The method of claim 3 wherein a rate of change in P_(L) orP_(H) indicates that said P_(L) or P_(H) is invalid.
 6. The method ofclaim 3 wherein the validity of said present P_(L) or P_(H) value isdetermined by a Threshold Adjustment Circuit (TAC) from one or more ofthe following parameters: the rate of chance in said P_(L) or P_(H), therelative change in said P_(L) or P_(H), the deviation between AVG and athreshold, one or more indicator values obtained via an activitydetector, and the domination of the other value said P_(L) or P_(H)where P_(H) and P_(L).
 7. The method of claim 6 wherein said indicatorvalues obtained via said activity detector, optionally in combinationwith said AVG or said threshold, allow the Threshold Adjustment Circuit(TAC) to determine a state of the signal.
 8. The method of claim 7wherein said states comprise two or more of the following possibilities:High activity Medium activity Low activity.
 9. The method of claim 8where a state of high activity causes said TAC to perform at least oneof the following determining the DC balance of the signal by determininga deviation between AVG and TH_(P), and adding suitable offset values toP_(L) and/or P_(H) optionally depending on said deviation, setting thethreshold according to the method applied during medium activity, ormaintaining constant value of said threshold.
 10. The method of claim 2wherein the determination of the second modulation or the secondmodulation index comprises the use of at least one peak detector. 11.The method of claim 10 where said peak detector comprises a differenceamplifier with a bandwidth BW_(P) having a cut off frequencycorresponding to the time period of one half bit or more, such as 1 bitor more, such as 2 bits or more, such as equal to 3 bits or more, suchas equal to 4 bits or more, such as equal to 5 bits or more, such asequal to 10 bits or more, such as equal to 20 bits or more.
 12. Themethod of claim 10 where said peak detector comprises a hold and/orreset function.
 13. The method of claim 10 where said peak detector hasa characteristic discharge time corresponding to the time span of 1 bitor more, such as 2 bits or more, such as equal to 3 bits or more, suchas equal to 4 bits or more, such as equal to 5 bits or more, such asequal to 10 bits or more, such as equal to 20 bits or more, such asequal to 30 bits or more, such as equal to 40 bits or more, such asequal to 50 bits or more, such as equal to 100 bits or more, such asequal to 10³ bits or more, such as equal to 10⁴ bits or more.
 14. Themethod of claim 2 comprising determining the second modulation index andsubsequently adjusting said decision threshold based on a determinationof the second toggling amplitude and the previously determinedmodulation index.
 15. The method of claim 2 comprising determining saidsecond modulation index and when said determined value is determined tobe valid and when at least one of P_(H), P_(L) and MA is determined tobe invalid, adjusting said decision threshold based on a determinationof the second toggling amplitude and its proportionality with the secondmodulation amplitude.
 16. The method of claim 2 wherein at least one ofthe P_(H) value or the MA is determined to be invalid in the event thatmore than 50% of the current signal over a time period longer than orequal to 10⁴ bits immediately after determining the P_(H) value has lowvalues relative to the TAC the bits.
 17. The method of claim 2 whereinat last one of the P_(L) or the MA value is determined to be invalid inthe event that more than 50% of the current signal over a time periodlonger than or equal to 10⁴ bits immediately after determining the P_(L)value has low values relative to the TAC.
 18. The method of claim 1comprising setting the second modulation index as a predetermined valueand adjusting said decision threshold based on a determination of thesecond toggling amplitude.
 19. The method of claim 1 comprisingadjusting a gain in said receiver circuit so that the second togglingamplitude is held substantially constant.
 20. The method of claim 1further comprising amplifying said IES thereby adjusting said secondmodulation amplitude and second toggling amplitude.
 21. The method ofclaim 1 wherein the setting of said threshold as a fraction of saidsecond modulation amplitude is based on an average (AVG) of the IES. 22.The method of claim 1 further comprising applying a limiting amplifier,said limiting amplifier having said threshold as one input and said IESas a second input and a limited version of IES, IES_(LIM), as output,measuring at least one signal parameter of IES and at least onecorresponding signal parameter of IES_(LIM) employing a feedback loop toadjust said threshold based on the difference between the measuredsignal parameter of IES and IES_(LIM) as an error signal.
 23. The methodof claim 22 further comprising determining a second signal parameter ofIES and a second corresponding signal parameter of IES_(LIM), andemploying a feedback loop to adjust the modulation amplitude ofIES_(LIM) or IES based on the difference between the measured secondsignal parameter of IES and IES_(LIM) as an error signal.
 24. The methodof claim 22 wherein said signal parameter is selected from P_(H), P_(L)and MA from IES.
 25. The method of claim 1 further comprising applying alinear differential amplifier, said differential amplifier having saidthreshold as one input and said IES or a derivative thereof as a secondinput and a differential version of IES, IES_(DIFF), as output, applyinga limiting differential amplifier having IES_(DIFF) as input andIES_(LIM) as output, measuring at least one signal parameter ofIES_(DIFF) and at least one corresponding signal parameter of IES_(LIM)employing a feedback loop to adjust said threshold based on thedifference between the measured signal parameter of IES_(DIFF) andIES_(LIM) as an error signal.
 26. The method of claim 25 furthercomprising measuring a second signal parameter of IES and a secondcorresponding signal parameter of IES_(LIM), and employing a feedbackloop to adjust the modulation amplitude of IES_(LIM) or IES_(DIFF) basedon the difference between the measured second signal parameter ofIES_(DIFF) and IES_(LIM) as an error signal.
 27. The method of claim 25wherein said signal parameter is selected from P_(H), P_(L) and MA fromIES.
 28. The method of claim 1 further comprising applying an activitydetector to indicate the activity in the electrical signal to obtain anindicator value for one or more of the following: number of transitionsbetween logic high and logic low within a predetermined time period,number of incidences of CIDs of logic high and/or logic low value longerthan a predetermined length over a predetermined time period.
 29. Themethod of claim 1 wherein said binary content of said optical signalcomprises long series of CIDs.
 30. The method of claim 29 wherein thelength of a series of CIDs of logic-high or logic-low is determineddisregarding series of CIDs of the other logic value shorter than aminimum length, such as shorter than or equal to 3 bits.
 31. The methodof claim 30 wherein a long series of CIDs is a period wherein two ormore of P_(H), P_(L) and MA is determined to be invalid.
 32. The methodof claim 29 wherein a long series of CIDs is a period wherein at leastone of P_(H), P_(L) and MA is determined to be invalid.
 33. The methodof claim 1 comprising applying an activity detector in said receivercircuit to determine whether said binary optical signal comprises longCIDs.
 34. The method of claim 1 wherein a calculated threshold iscompared to the average of the electrical signal to determine thevalidity of P_(L) or P_(H) such as a threshold calculated as(P_(L)+P_(H))/x, where P_(H) and P_(L) are optionally adjusted for anoffset relative to said binary electrical signal.
 35. The method ofclaim 1 where a substantial deviation between an average of the IES,AVG, and a calculated threshold, TH_(P), indicates that either P_(L) orP_(H) is invalid, so that a. when AVG>TH_(P) then P_(L) is deemedinvalid and b. when AVG<TH_(P) then P_(H) is deemed invalid.
 36. Themethod of claim 35 where a substantial deviation calculated as$100{\% \cdot {{2\frac{{AVG} - {TH}_{P}}{P_{H} - P_{L}}}}}$ is largerman or equal to 5%.
 37. The method of claim 1 further comprisingconverting said IES in said receiver circuit into a binary electricalsignal output downstream of said input via said decision threshold. 38.The method of claim 1 wherein said binary content of said optical signalhas a bit frequency and said toggling signal has a center frequency ofless than 10% of said bit frequency, such as less than 5% of said bitfrequency, such as less than 2.5% of said of said bit frequency, such asless than 1% of said bit frequency, such as less than 0.5% of said bitfrequency, such as less than 0.1% of said bit frequency, such as lessthan 0.01% of said bit frequency.
 39. The method of claim 1 wherein saidmodulation amplitude is determined as an average value over a period ofsaid toggling signal.
 40. The method of claim 1 wherein said modulationindex is defined as the ratio of said toggling signal relative to anaverage value of said modulation amplitude over a period of saidtoggling signal.
 41. The method of claim 1 wherein said signal has a bitfrequency larger than 100 Mbit, such as larger than or equal to 1 Gbit,such as larger than or equal to 10 Gbit, such as larger than or equal to25 Gbit, such as larger than or equal to 50 Gbit, such as larger than orequal to 100 Gbit.
 42. The method of claim 1 wherein said threshold isupdated by adjusting one or more of a shunt current source suitable forshunting part of the current from the photo detector, an offset for anamplifier, such as an amplifier converting a current from the photodetector to a voltage, and the threshold for a decision circuit, such asa limiting amplifier.
 43. The method of claim 1 wherein determination ofsaid second toggling amplitude comprises the use of a full waverectifier circuit.
 44. The method of claim 43 wherein said full waverectifier circuit comprises differential amplifier having said decisionthreshold as one input.
 45. The method of claim 44 wherein saiddifferential amplifier is a linear amplifier.
 46. The method of claim 1wherein the values P_(L) and P_(H) are obtained relative to a thresholdvalue.
 47. An optical receiver circuit for receiving an optical signalwith a binary content and converting it to an intermediate electricalsignal, IES, with a binary content comprising a second logic-high and asecond logic-low defining a second modulation amplitude, and a sub-bandmodulation of said second modulation amplitude having a togglingamplitude, the optical receiver comprising a circuit arranged to measurean toggling amplitude of said second toggling signal, and a circuitarranged to adjust said decision threshold based on the second togglingamplitude.
 48. The circuit of claim 47 further comprising a full waverectifier circuit having a decision threshold as one input.
 49. Thecircuit of claim 48 wherein said full wave rectifier comprises adifferential amplifier having said decision threshold as one input andsaid voltage signal as another input.
 50. The circuit of claim 47further comprising a feedback loop arranged to maintain substantialconstant toggling amplitude.
 51. The circuit of claim 47 furthercomprising a peak detector for determine a value relating to the secondlogic-low and/or the second logic-high level of said voltage signal. 52.The circuit of claim 47 wherein said circuit arranged to determine thetoggling amplitude comprises a full wave rectifier circuit.
 53. Thecircuit of claim 47 further comprising circuitry for implementing themethod of optical transmission of an optical signal comprising a binarycontent comprising transmitting the optical signal with the binarycontent comprising a first logic-high and a first logic-low defining afirst modulation amplitude wherein the first modulation amplitude issub-band modulated with a toggling signal having a first togglingamplitude with a first modulation index relative to said firstmodulation amplitude, receiving said optical signal with an opticalreceiver circuit and converting the optical signal to an intermediateelectrical signal, IES, said IES having the following parameters: iii. asecond logic-high and a second logic-low defining a second modulationamplitude and iv. a second toggling amplitude having a second modulationindex relative to said second modulation amplitude, providing a decisionthreshold relative to said IES as a function of said second modulationamplitude, and adjusting the decision threshold by determine the secondtoggling amplitude and adjusting said decision threshold relative tosaid IES based on proportionality between the second toggling amplitudeand said second modulation amplitude.
 54. An optical receiver comprisinga receiver circuit according to claim 47 connected to a photodiode.